1. Field of the Invention
The present invention relates to a valve train for an internal combustion engine, provided with a hydraulic rocker arm interlocking mechanism for selectively engaging a plurality of rocker arms or disengaging the plurality of rocker arms.
2. Description of the Related Art
Referring to FIGS. 4 and 5 showing a valve train for an internal combustion engine of this type disclosed in JP-A13443/1999, the valve train includes a pair of intake valves a1 and a2 for each of a plurality of cylinders of a multiple-cylinder internal combustion engine, a first rocker arm b1 for driving the intake valve a1, a second rocker arm b2 for driving the intake valve a2, a free rocker arm b3 capable of being independent of the intake valves a1 and a2, a rocker shaft, and a hydraulic rocker arm driving mode selecting device d. The rocker arms b1 and b2 are provided with openings g1 to g3 in parts thereof between tappet crews e1 and e2 to be engaged with the valve tips of the intake valves a1 and a2 and the rocker shaft c to receive rollers f1 to f3. Cam lobes of a camshaft p come into rolling contact with the rollers f1 to f3. The rollers f1 to f3 are supported for rotation on a tubular shaft k1 supported on a first support wall h1 and a second support wall h2, a tubular shaft k2 supported on a first support wall h3 and a second support wall h4, and a tubular shaft k3 supported on a first support wall h5 and a second support wall h6, respectively. A pressure chamber n is formed in the first rocker arm b1 between the first support wall h1 and a timing piston m is slidably fitted in the tubular support shaft k1. The first support wall h1 is provided with a connecting passage s having one end opening into the pressure chamber n to into connect an oil passage r formed in the rocker shaft c and the pressure chamber n. The other end of the connecting passage s forms a through hole s0 extending from a bore t, in which the rocker shaft c is fitted, through a side part of the first rocker arm b1. The tappet screws e1 and e2 are supported on substantially entire part of walls forming tip parts of the rocker arms b1 and b2 including end parts of the support walls h1, h2, h3 and h4, and outer walls w1 and w2 on the radially outer side of the rollers f1 and f2.
In this prior art valve train, the connecting passage s having one end connected to the pressure chamber n and the other end connected to the through hole s0 and formed in a support part u of the first rocker arm b1, forming the bore t in which the rocker shaft c is fitted reduces the rigidity of the support part u. The tappet screws e1 and e2 can be sufficiently rigidly supported by substantially entire end parts of the rocker arms b1 and b2 when the rollers f1 and f2 of the prior art valve train are comparatively thin. When tappet screws are supported by substantially entire end parts of rocker arms in a valve train having comparatively thick rollers, the weight and inertia of the rocker arms increases. Although the rocker arms can be formed in lightweight structures if the tappet screws are supported only by the outer side walls, it is difficult to support the tappet screws highly rigidly because the supporting effect of the support walls is low.
The present invention has been made in view of such problems and it is a first object of the present invention to provide a valve train for an internal combustion engine, including a small, lightweight rocker arm provided with a pressure chamber, and a connecting passage connecting the pressure chamber and an oil passage formed in a rocker shaft, and having a rigid support part to be supported on the rocker shaft.
Another object of the present invention is to provide a valve train for an internal combustion engine, including a roller-type rocker arm having a rigid contact tip to be brought into contact with a valve of the internal combustion engine.
According to the present invention, a valve train for an internal combustion engine, comprises: a rocker shaft provided with an oil passage; a plurality of rocker arms having support parts provided with bores to receive the rocker shaft therein so as to be supported on the rocker shaft for rocking motion and operated for rocking motion by cams to operate valves of the internal combustion engine, specific one among the rocker arms being provided with a pressure chamber, a straight connecting passage formed in one specific end part thereof so as to extend obliquely to a direction parallel to an axis of the bore of the support part of the same specific rocker arm so as to be connected to an oil passage formed in the rocker shaft; and a rocker arm interlocking mechanism including a connecting piston fitted in the pressure chamber of the specific rocker arm so as to be axially displaced by pressure of working fluid supplied into the pressure chamber of the specific rocker arm, and capable of changing the position of the piston in the pressure chamber to engage the plurality of rocker arms or to disengage the plurality of rocker arms; wherein an open end surface of the bore of the specific rocker arm on the side of the specific end part is on an axially outer side of the pressure chamber; the linear connecting passage is inclined to the axis of the bore of the specific rocker arm so that an imaginary passage on an extension of the connecting passage extended toward the open end surface of the bore of the specific rocker arm intersects the open end surface; the outermost side surface of the specific end part has an inclined section inclined to the axis of the bore of the specific rocker arm along the connecting passage.
According to the present invention, the imaginary passage on the extension of the straight connecting passage formed in the specific end part of the specific rocker arm among the plurality of rocker arms is inclined to the axis of the bore so as to intersect the open end surface of the bore on the side of the specific end part. Therefore, any through hole is not formed in the support part opposite to the connecting passage with respect to the bore even when the connecting passage is formed by machining using a drill. Thus, the support part has a high rigidity and the specific rocker arm rocks stably when the internal combustion engine operates at high engine speeds. Since the open end surface of the bore formed in the support part projects axially beyond the pressure chamber, the connecting passage extends from the side of the support part toward the pressure chamber backward obliquely to the axis, and an outermost end surface extends obliquely backward along the connecting passage. Therefore, the specific rocker arm is miniaturized and the weight of the same is reduced accordingly. Since a space of a size corresponding to the retraction of the outermost side surface along the connecting passage is formed axially outside the outermost side surface of a part corresponding to the pressure chamber of the specific rocker arm, component parts of the valve train can be disposed in the space so that the rocker arms and the associated parts can be compactly arranged.
In the valve train according to the present invention, at least one of the plurality of rocker arms is a roller-type rocker arm having first and second support walls and a outer side wall defining an opening for holding a roller to be brought into rolling contact with the cam, the first and the second support wall are joined to the outer side wall at two corner parts, respectively, the first and the second support wall disposed axially opposite to each other hold opposite end parts of a support shaft supporting the roller for rotation, the outer side wall is disposed on the outer side of the roller with respect to a direction parallel to a diameter of the bore, and the contact part to be brought into contact with the valve of the internal combustion engine is supported on one of the corner parts.
Since the contact part of the roller-type rocker arm is supported on the corner part having a rigidity higher than that of the outer side wall forming the opening because the highly rigid first support wall is connected to the outer side wall, increase in size and weight to enhance the rigidity of the part of the rocker arm for supporting the contact part can be avoided. This effect is remarkable when a thick roller is employed.
In the valve train according to the present invention, the specific rocker arm may be a roller-type rocker arm, the specific axial end part may include the first support wall, the connecting passage may be formed in the first support wall, the first support wall may have a width along the axis of the bore greater than that of the second support wall, and the contact part may be supported on the corner part where the first support wall and the outer side wall are joined.
Since the first support wall provided with the connecting passage and having the width greater than that of the second support wall is joined to the outer side wall at the corner part supporting the contact part, the corner part supporting the contact part has a high rigidity, and, consequently, the valves of the internal combustion engine can be operated more stably when the internal combustion engine operates at high engine speeds. Since a part of the support part on the side of the contact part projects, pressure exerted on the rocker shaft by the support part resulting from a torsional moment produced by a force exerted on the roller by the cam and a force acting on the contact part can be reduced, which stabilizes the operation of the valves of the internal combustion engine.
In the valve train according to the present invention, the specific rocker arm is preferably a roller-type rocker arm, and the bore may have a wall surface including a circumferentially extending annular raised portion which protrudes radially inwardly of the bore and extends axially of the bore in a region between the connecting passage and the opening. This feature of the interior wall surface of the bore provides an advantage of increasing the rigidity of the first rocker arm.
In the valve train, the bore may have a wall surface including a circumferentially extending annular recessed portion which recedes radially outwardly of the bore and extends axially of the bore in a region corresponding to the opening. This feature of the interior wall surface of the bore provides an advantage of reducing the weight of the first rocker arm.
In the valve train, the bore may have wall surface portions extending in axial regions of the bore in which the first and second support walls are formed, respectively, the wall surface portions having a diameter smaller than that of a wall surface portion of the bore extending in an axial region in which the opening is formed. This feature of the interior wall surface portions of the bore provides an advantage of excellent lubrication in the mutually sliding portions of the rocker shaft and the first rocker arm.